The present invention relates generally to photosensitive bulk glass, and particularly to meltable alkali boro-alumino-silcate and germanosilicate glasses.
Optical transmission systems, including optical fiber communication systems, have become an attractive alternative for carrying voice and data at high speeds. While the performance of optical communication systems continues to improve, there is increasing pressure on each segment of the optical communication industry to reduce costs associated with building and maintaining an optical network.
Quite often, optical communication systems require various types of optical filter elements. For example, diffractive filter elements may be used to effect the separation (demultiplexing) of individual wavelength channels in a wavelength division multiplexed (WDM) optical system. In addition, these refractive filter elements may be used to compensate for the ill-effects of dispersion, to include chromatic dispersion (CD) and polarization mode dispersion (PMD).
One type of diffractive optical filter is a Bragg grating. Bragg gratings are interferometric optical devices which have been employed in a variety of applications including multiplexing/demultiplexing applications and dispersion compensation applications. Bragg gratings may be used to reflect light of a wavelength which satisfies the Bragg phase matching condition, and which transmits all other wavelengths.
One useful technique for forming a Bragg grating is to selectively alter the index of refraction in a substrate in a periodic manner. This selective alteration of the index of refraction can be used to fabricate both Bragg gratings in which the period of the index of refraction is regular, as well as chirped gratings in which the period of the index of refraction varies as a function of distance.
What is needed is a batch meltable glass material which is photosensitive, and which overcomes certain drawbacks of conventional glass materials.
The invention includes a photosensitive glass. According to an exemplary embodiment of the present invention a meltable photosensitive glass has a hydrogen content of greater than 1017 hydrogen molecules/cm3. The refractive index change in the exposed portions of the glass is 10xe2x88x924 (xcex94n  greater than 10xe2x88x924) measured at a wavelength of 633 nm; and the glass is photosensitive to light having a wavelength of less than 300 nm.
According to another exemplary embodiment of the present invention, the starting glass is a photosensitizable alkali boro-alumino-silicate glass that can be loaded with hydrogen to make it photosensitive. In one exemplary embodiment of the present invention, the glass is a below 300 nm photosensitive glass which has a composition of 40-80 mole % SiO2, 2-15 mole % GeO2, 10-36 mole % B2O3, 1-6 mole % Al2O3 and 2-10 mole % R2O where R is chosen from the alkali elements with the glass exhibiting photosensitivity to below 300 nm wavelengths. In another exemplary embodiment of the present invention, the glass has a composition including approximately 25 weight % to approximately 45 weight % SiO2, approximately 3 weight % to approximately 22 weight % GeO2, approximately 7 weight % to approximately 28 weight % B2O3, approximately 6 weight % to approximately 22 weight % Al2O3, approximately 6 weight % to approximately 25 weight % R2O wherein R is an alkali, and approximately 3 weight % to approximately 11 weight % F, with the glass exhibiting photosensitivity to below 300 nm wavelengths.
Another exemplary embodiment of the present invention includes a molecular hydrogen loadable photosensitive bulk glass. The photosensitive bulk glass is an alkali boro-alumino silicate glass with a melting temperature no greater than 1650xc2x0 C. Preferably, the glass has a batch composition comprising no greater than 85 mole % SiO2, no less than 10 mole % B2O3, no less than 2 mole % GeO2, and a combined alkali and alumina content no greater than 20 mole % Al2O3+alkali with the glass having a molecular hydrogen loadable level of at least 1018H2 molecules/cm3.
Another exemplary embodiment of the present invention includes a meltable photosensitive germanosilicate glass material having a hydrogen content less than 1017H2 molecules/cm3. In one exemplary embodiment of the invention, the glass is a below 300 nm photosensitive glass which has a composition of 40-80 mole % SiO2, 2-15 mole % GeO2, 10-36 mole % B2O3, 1-6 mole % Al2O3 and 2-10 mole % R2O where R is chosen from the alkali elements with the glass exhibiting photosensitivity to below 300 nm wavelengths. In another exemplary embodiment of the invention, the glass includes approximately 25 weight % to approximately 45 weight % SiO2, approximately 3 weight % to approximately 22 weight % GeO2, approximately 7 weight % to approximately 28 weight % B2O3, approximately 6 weight % to approximately 22 weight % Al2O3, approximately 6 weight % to approximately 25 weight % R2O wherein R is an alkali, and approximately 3 weight % to approximately 11 weight % F, with the glass exhibiting photosensitivity to below 300 nm wavelengths.
Another exemplary embodiment of the present invention includes a method of making a refractive index pattern. The invention includes providing a photosensitive bulk glass having a 300 nm absorption less than 20 dB/cm, providing a radiation source below 300 nm, forming a pattern with the below 300 nm radiation, and exposing the photosensitive bulk glass to the pattern to form a modulated refractive index pattern in the bulk glass.
Another exemplary embodiment of the present invention includes a method of making a molecular hydrogen loadable photosensitive glass optical device preform. Preferably, the method comprises making a refractive index pattern preform out of melted glass. The method includes providing a germania silica glass powder batch with transition metal contamination level xe2x89xa61 ppm by heavy metals. The method includes melting contamination level xe2x89xa61 ppm by weight for heavy metals. The method includes melting the silica glass powder batch to form a homogeneous glass melt, cooling the glass melt into a UV transmitting bulk glass having a 300 nm absorption less than 20 dB/cm and forming the bulk glass into an optical device preform in which refractive index patterns can be made.
Another exemplary embodiment of the present invention includes a photosensitive glass optical refractive index pattern preform for use with UV light in the formation of refractive index patterns. The preform is comprised for use with UV light in the formation of refractive index patterns. The preform is comprised of an alkali boro-alumino-silicate glass with a 300 nm absorption less than 20 dB/cm. The preform glass has a UV wavelength inducable modulated refractive index xcex94n level greater than 10xe2x88x925 with a molecular hydrogen level of at least 1018H2 molecules/cm3.